The invention relates to an apparatus having a component, which may mechanically vibrate, wherein the mechanical vibration is undesirable and is therefore to be damped.
One example of such an apparatus is brakes. Although the invention is described below with regard to brakes, the invention may also be used in other apparatuses. Mechanical vibrations occur particularly in apparatuses, in which moving parts rub against one another and set the system into undesired vibrational states.
Said phenomenon has long been familiar in brakes. Mechanical vibrations in brakes, such as e.g. disk brakes or also drum brakes, frequently lead to highly undesirable sound radiation, so-called xe2x80x9cbrake squealingxe2x80x9d. For some time attempts have been made to theoretically elucidate the causes of brake squealing and combat them through practical measures.
Thus, U.S. Pat. No. 974,549, for example, proposes the provision of drill holes in steel wheels, gears or also other wheels in order to interrupt a vibration transmission path. In addition, it is proposed that a vibration-damping material, such as lead, be disposed in the bores.
U.S. Pat. No. 1,572,680 sets out to reduce noise generation in a drum brake by providing the anchor plate in a specific region with recesses or openings, which are filled with a specific material selected so as to inhibit vibrations of the anchor plate.
GB-PS 1381589 sets out to prevent undesirable noise generation in a fixed-caliper disk brake by using a mass coupling to relocate centres of gravity.
Prior art also includes the proposal from DE 195 05 000 A1, in the case of a floating frame brake, to dampen vibrations of the frame by providing a xe2x80x9cvibration absorberxe2x80x9d surrounded by silicone oil.
U.S. Pat. No. 4,445,594 sets out to avoid vibrations by fastening and supporting the outer brake pad of a partially lined floating-caliper disk brake via rubber elements in the bridge fingers.
JP-A 59-200819 also teaches that the bridge fingers of a disk brake be provided with vibration dampers. The vibration dampers are made of rubber or a plastics material and steel masses are moreover purposefully attached. By virtue of different natural frequencies of the bridge, on the one hand, and of the said masses, on the other hand, an attempt is made to combat vibration of the system and/or of its components.
EP-A 0 592 290 sets out to dampen vibrations in disk brakes by bracing a clamping wedge, on the one hand, with a side wall of the housing of the disk brake and, on the other hand, with a flange of the axle, to which the disk brake is fastened. In addition, an elastomeric layer may be provided between the clamping wedge and the side wall.
Further attempts of an extremely diverse nature at preventing undesirable mechanical vibrations in brakes are described e.g. in WO95/19508, EP 0 456 301 B1, EP 0 455 299 B1 and DE-GM 7801919.
For the suppression of vibrations in an internal combustion engine, EP 0 470 064 A2 discloses a device comprising a piezoelectric sensor element, which detects structure-borne sound vibrations, and an actor element, which generates structure-borne sound vibrations in phase opposition. In said case, signals of the sensor elements are supplied to the actor element via an automatic electronic control device, which also comprises an amplifier module for the supply of external energy.
The object of the invention is to provide simple, inexpensive and functionally reliable means of effectively preventing the mechanical vibrating of a component of an apparatus.
For solving said technical problem the invention proposes an apparatus and/or a brake having the features of claims 1, 2 and 4.
As is known, piezoelectric elements may deform upon application of a voltage and so, when a periodic voltage is applied, execute a mechanical vibration, i.e. for example, periodically deform. According to the invention, said mechanical vibration of the piezoelectric element is then used in such a way that it counteracts the vibration of the component of the apparatus which is to be damped. According to the fundamental rules of physics this may occur, for example, by producing a suitable phase relationship between the mechanical vibration of the component to be damped and the deformation vibration of the piezoelectric element. In said case, the vibration of the component and the vibration of the piezoelectric element need not necessarily have the same frequency, rather a relationship between the frequencies according to whole-number multiples (xe2x80x9charmonicxe2x80x9d) is generally sufficient to achieve a damping effect.
The refinement of the invention according to claim 2 offers a number of advantages. First of all, a piezoelectric element is used in order, by means of the mechanical vibration of the component to be damped, to generate the voltage which is applied to another piezoelectric element (or alternatively even the same piezoelectric element, see below) in order to produce the mechanical vibration of the piezoelectric element (xe2x80x9celectrostrictionxe2x80x9d) and hence dampen the undesired vibration of the component. Such a system is self-regulating in the sense that the vibration-damping periodic vibration of the one piezoelectric element arises through voltage generation at the other piezoelectric element only when the vibration-endangered component of the apparatus actually vibrates (which need not necessarily always be the case). In said variant of the invention, the periodic electrical excitation of the vibration-damping piezoelectric element diminishes (i.e. the applied voltage reduces) or is completely interrupted when the mechanical vibration of the component to be damped diminishes or stops. The system does not even require external energy (voltage) to become effective, although the use of external additional sources of electrical energy for amplification purposes may be provided in more complex systems.
The previously described embodiment of the invention does not necessarily require two different piezoelectric elements, the teaching alternatively being realizable e.g. according to claim 1 with a single piezoelectric element, which both generates a voltage piezoelectrically as a result of being acted upon by the mechanically vibrating component of the apparatus and is suitably excited by said generated voltage itself with such phase displacement that it continues to vibrate in a state, in which it damps the mechanical vibration of the component of the apparatus. In said embodiment of the invention, therefore, the piezoelectric element vibrates in two states (modes): firstly, it is periodically deformed by the mechanical vibration of the component to be damped and so, because of the piezoelectric effect, periodically generates a voltage and, secondly, said voltage itself, optionally after suitable phase displacement and/or modulation, is reapplied for the purpose of electrical excitation to the same piezoelectric element in order to set the latter into a state of electrostrictive periodic mechanical vibration, as a result of which the mechanical vibration of the component of the apparatus is damped. Here too, as described above, the damping is self-regulating: if no component of the apparatus is vibrating, there is also no piezoelectric voltage generated and, conversely, also no electrostrictive vibration generated for damping (with the result that an undesired vibration triggered by the piezoelectric crystal itself is avoided). Said self-regulation of the system is proportional, i.e. the quantity of the damping vibration of the crystal depends upon the quantity of the undesired mechanical vibration of the component of the apparatus which is to be damped.
According to a preferred refinement of the invention it is provided that by means of an inductor a phase relationship is adjusted between the generated periodic voltage and the voltage applied for damping purposes.
A further preferred refinement of the invention provides that at least one piezoelectric element is inserted as a capacitive element in at least one resonant electric circuit, and that said resonant circuit has a resonant frequency, which is tuned to the mechanical vibration frequency which is to be damped.
In said case, at least two piezoelectric elements are preferentially connected either in parallel or in series in a resonant circuit having an inductor in such a way that the resonant frequency of said resonant circuit is substantially tuned to the mechanical vibration frequency of the component to be damped. In said case, in the previously described sense the one piezoelectric element may because of the piezoelectric effect generate a voltage, which is applied to the other piezoelectric element in order on the basis of electrostriction (i.e. the reverse piezoelectric effect) to effect mechanical vibration of said piezoelectric element and hence damping of the vibration of the component of the apparatus. The piezoelectric elements therefore operate reciprocally (crosswise) and are identical in their functions.
The invention relates not only to apparatuses of the type discussed here but also to methods of damping components in apparatuses, wherein the method realizes the previously described teachings.
As has already been mentioned above, the particularly preferred field of application of the invention is brakes, in particular disk brakes, such as floating-caliper or floating-frame brakes, or alternatively drum brakes.
In said case, it is preferentially provided that the brake is a floating-caliper or floating-frame brake and that the component, which may generate noises, is a pad carrier and/or a part of the caliper, such as a bridge finger.
In said case, it is preferentially provided that at least one piezoelectric element acts between the pad carrier and a part of the caliper and/or frame of the brake.
Preferentially, at least two piezoelectric elements are disposed in such a way that they are spaced apart from one another in peripheral direction of the brake (i.e. in the direction of rotation of the brake disk; i.e. tangentially). In said case, in relation to a radial centre plane of the disk brake the preferred arrangement may be one piezoelectric element on the run-in side and another piezoelectric element on the run-out side of the brake. Measurements have shown that with such an arrangement in a disk brake a particularly effective damping of the mechanical vibrations of the system is achievable.
As explained above, preferred refinements of the invention provide that the piezoelectric element or the piezoelectric elements are inserted in an electric circuit in order to transmit voltages and give rise to the described feedback effect. For phase-angle adjustment an inductor, i.e. a component with an inductive effect, may in said case be provided in the electric circuit. According to a preferred refinement of the invention, the inductor may be integrated directly in a component of the brake, e.g. the component to be damped, or alternatively mounted on such a component of the brake, e.g. in the immediate vicinity of the piezoelectric element, so that it may be mounted together with the latter.
Given the use of two or more piezoelectric elements in a resonant circuit, the latter is preferentially tuned in such a way that the damping effect of at least one of the piezoelectric elements is at its maximum in the region of a mechanical vibration frequency of the component of the apparatus which is mainly to be damped.
In a brake, the at least one piezoelectric element is preferentially disposed in the force flux of a brake application force of the brake because the friction-induced vibrations arise in said force flux path and hence may also be effectively damped there.
The invention may be realized not only generally in apparatuses and in particular brakes of the described type but also in individual parts of such apparatuses and/or brakes, such as the following.
Thus, a brake pad for a disk brake may comprise a piezoelectric element for vibration damping purposes. In said case, the piezoelectric element or elements may be fastened to, or integrated in, the carrier plate of the brake pad. It is even possible to design the carrier plate of the brake pad as a whole as a piezoelectric element or to fashion a part of the carrier plate as a piezoelectric element.
Analogous applications of the inventive idea described above for brake pads are possible in the case of brake shoes 70 for drum brakes 72 shown in FIG. 13, actuating pistons for brakes, brake carriers for disk brakes and anchor plates 74 for drum brakes, which may therefore be in each case entirely or partially of a piezoelectric design or may comprise one or more piezoelectric elements.
To achieve effective damping of the mechanical vibration of the component of the apparatus or of the components of the apparatus, according to a further preferred refinement of the apparatus according to the invention it is provided that at least one piezoelectric element is disposed close to an antinode of the mechanically vibrating component. An antinode in the present context is a point, at which the mechanically vibrating component vibrates at maximum amplitude. An anti-periodically vibrating piezoelectric element acting at said point produces a particularly high damping effect.